Laser printers are based on the principle of electrophotography. By means of imaging or illumination of a photoconductor with the optical image of an original, electrophotography produces a latent image from electrical charges, which will then be used to selectively apply a toner (development) and to image (copy) the original, for example onto paper. A distinction is made between direct and indirect as well as wet and dry electrophotography. The wet methods, also referred to as liquid toner methods, use a dispersion of the toner in an aliphatic solvent with low electric constant and as developer, whereas the dry method uses a powder toner. An image of the desired page is recorded onto the light-sensitive imaging drum with the aid of a bundled laser beam and a rotating mirror. The drum is initially charged negatively, wherein the charge is removed again at the points at which the laser beam impinges. The shape of the discharged areas on the drum corresponds to the subsequent printout. The toner is applied to the drum via a roller with negatively charged toner, which remains adhered to the discharged points on the imaging drum.
With the dry toner method, the paper is then guided over the imaging drum. It only touches the drum gently. A electrical field is formed behind the paper. The toner is transferred to the paper, where it adheres initially only loose. The toner is then fixed with the aid of a hot roll and under pressure. The drum is discharged and excess toner is removed therefrom.
With the liquid toner method, the toner suspension is initially transferred to a heated rubber cylinder, on which the carrier liquid is evaporated and the toner is plasticised. The toner image is then transferred to the recording sheet from this intermediate roll (intermediate drum).
The images produced with the aid of a laser printer should achieve a quality comparable to that of a photo. This includes properties such as gloss, stiffness, opacity, high resolution and image sharpness as well as good light fastness.
In the publication by HP (Hannelore Breuer): Printing Know-How: New HP Laser Papers of May 13, 2005, available at 41131.www4.hp.com/Backgrounder_Neue_Laser-Papiere.pdf (as of Aug. 31, 2010), multi-layer coated papers with an “open” pore surface on either side are described. However, images produced with use of such papers have a surface feel that is quite different from conventional silver halide images and a different surface gloss.
In order to come closer to the objective of photo-like quality, images produced electrophotographically are produced on carrier materials that have the surface feel and the appearance of a typical silver salt photo. DE 44 35 350 C1 describes an image receiver material for electrophotography, which comprises a base paper coated with thermoplastics and a toner receiver layer and also an antistatic rear layer. A disadvantage of this material is that it can be improved further with regard to the toner fixing and the behaviour in the printer. Furthermore, once an image has been printed, such materials sometimes show bothersome glossy spots, which are caused by the oil-like substances often used as separating agents in the toner formulations.
DE 10 2011 078 714 A1 describes recording materials with toner receiving layers, which contain fine inorganic particles and which have pores of a pore radius of from 100 nm or less. The application of such materials is restricted to liquid toner methods as is explained in the document. During further processing of the print outs on such porous materials, for example to obtain a photo book, the danger exists that the porous toner receiving layer breaks along the feather edge of the print out and, as a consequence, an undesired white line at is formed at the break line.
Further recording materials for electrophotographic methods are described for example in EP 0 789 281 B1, EP 1 115 559, JP 2006-215 494, JP 2007-188 055 and JP 2010-020 283.